Those are not postulates they are enthymemes.
Morality is relative is a postulate. TO deduce the conclusion that anyone can believe anything you need to supply a premise. But actually what you WANT TO DO is to postulate that anyone can believe anything directly.
In short, these are a mess as the basis for discussion.
Further you say things that contradict what you seem to be aiming at.
One is also able to tell oneself lies too. Right? And the immorality of many lies is in the fact that you weren't able to tell the truth even if you wanted to -- BECAUSE you were to lazy to get to the truth in the first place.